weird noise from STA321MP output

son1 · ‎2016-01-07

Posted on January 07, 2016 at 11:04

Hi,

I am using STA321MP to record the audio signal, I have observed the noise from its output signal when there is no speech. The period is ~20ms. Do you have any idea where it is from?

Nickname1623_O · ‎2016-01-07

Posted on January 08, 2016 at 07:23

hi Phan,

for STA321MP , you are able to control volume of output channel , please use -80dB to mute output , then check the output noise again.

or you can disconnect microphone PDM output data signal line , then check the output again.

Thanks

YT

son1 · ‎2016-01-08

Posted on January 08, 2016 at 11:21

Hi,

Thank for the clever suggestion!

I do the same and the noise is showed clearly.

Nickname1623_O · ‎2016-01-10

Posted on January 11, 2016 at 07:14

Hi Phan,

Could you let me know the signal path after STA321MP ?

did you send I2S output to a DAC ?

if yes, please let me know your clock arrangement for system and I2S bus .

Thanks ,

YT

son1 · ‎2016-01-11

Posted on January 11, 2016 at 13:11

Hi Tao,

I am using STM32F7-DISCO board to read SDOxx from STA321MP via I2Sx and SPIx channels. The reading data is streamed to codec chipset WM8994 via SAI to play-out the audio signal. The clock for I2S/SPI bus is ~1.025Mhz (16Khz*32bit*2 audio channels).

The clock for XTI is supplied from external oscillator 12.288 Mhz.

Is the noise caused by my current circuit? I see It correlates with power line signal(50hz)

I am building my own board, I hope that kind of noise will not occur.

Nickname1623_O · ‎2016-01-11

Posted on January 12, 2016 at 02:21

Hi Phan,

it doesn't look like 50 hz noise from AC power supply , but some digital noise , like pop corn.

according to your discription, the MCLK for STA321MP had been provided from external clock source and freq is 12.288Mhz , did you also use this osc for STM32 ? due to STA321MP was a slave device, the I2S clock ( LRCK and BICK) should be provided from STM32 ( as master ), if MCLK was not synchronized with I2S clock , it will lead to pop corn noise in audio band .

Thanks ,

YT

son1 · ‎2016-01-11

Posted on January 12, 2016 at 05:09

Thank for the comments! I got your point.

STM32 is using 25Mhz osc. They are not the same clock now. I tried to provide MCLK to STA321MP from STM32 last time. But for some reasons, it didn't work last time. Now, I will revert to that option and check but I don't think it is the root cause. Since when I removed Microphones or set MUTE for STA321MP's output, I get same noise signal. As far as my understanding, all audio signal is 0 would work without noise even the clock is not synchronize. Please correct if I am wrong.